1. Field of the Invention
This invention relates to a location system for use in a mobile communication network such as a cellular communication system.
2. Description of the Related Arts
A number of different mechanisms have been proposed to determine locations of mobile units within a cellular communication network. One of the most commonly used is known as EOTD (enhanced offset time differential, or enhanced observed time differential). This system works by measuring at a mobile unit, the time difference between specific signals arriving from different base stations. Three or more base stations are required for the measurement. This produces an offset between the signals from the different base stations and the offset is reported back to the network. Appropriate processing within the network can then compare the offset with equivalent data from a measurement made at a known fixed location. This comparison then allows the location of the handset to be calculated in relation to the base stations and the known fixed location.
FIG. 1 shows a mobile communication network that comprises three base Stations 24. Two types of transmitting and receiving units, that is, a mobile handset 24 and a fixed position unit 26, receive signals from the base stations. Each of these transmitting and receiving units measures the time difference between known signals from each base station. Each transmitting and receiving unit reports this offset data to the network.
The network knows the locations of all the base stations 20 and of the fixed position unit 26. It is possible to calculate the distance of the mobile handset 24 from each of the three base stations 20 using the offset data. The mathematics required to perform this calculation is well known and need not be reproduced here but will be familiar to those skilled in the art.
The output of the calculations is a series of arcs describing the separation between the mobile handset and each base station. There is an inherent error in each arc due to factors such as the accuracy of the measurement. The location of mobile handset 24 is therefore determined as the center of the overlap area of the arcs, subject to an uncertainty of the size of the overlap area.
The fixed position unit (FPU) 22 can have the data it receives combined with data from a number of other FPUs to derive a map of timing differences over an area. By comparing the measurement data from the mobile units, that is, mobile handsets, and the timing map derived from the FPU, an estimate of the handset location can be obtained.
There is problem with this in that FPUs are generally placed on sites which are already owned by the network (i.e., base station locations). This limits the number of sites and gives a somewhat simplistic view of the propagation of radio waves. EOTD uses the data from these to model the timing differences expected within the area. However, this assumes linearity of timing differences between the known FPUs. This is accurate for direct line propagation but regrettably such propagation is uncommon in cellular systems; reflection and refraction of signals by buildings, hills etc. are significant factors in altering the distance traveled by a radio signal. This leads to differences between the model and reality and thus limits the accuracy of the location determined for the mobile handsets or handheld portables (HHP).
To date, two proposals have been made to try and remove these errors. These are as follows:
(1) Using geographical information system (GIS) to provide data on hills, buildings etc., in the area. Then the propagation modeling is used to deduce the likely effect of the obstacles present. The modeling required is processor intensive and is only valid for the atmospheric conditions defined. Furthermore, the results are all dependant on visible features being in the GIS system. Any additional building or demolition requires the models to be rerun.
(2) To increase the number of FPUs and thus give more reference locations for the model to work with. From the network point of view this is an expensive solution for little benefit, it involves purchases of extra hardware locations but the accuracy of the locations reported increases only with the square root of the number of FPUs.